This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2002-223521, filed Jul. 31, 2002, the entire contents of which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to a run length limited code generation method for generating a code used to adjust a reproduction circuit, which reproduces digital data from an information storage medium. The present invention also relates to a run length limited code recording/reproduction apparatus, which generates a code used to adjust a reproduction circuit, which reproduces digital data from an information storage medium, and records/reproduces that code on/from an information storage medium. Furthermore, the present invention relates to a run length limited code recording/reproduction method for generating a code used to adjust a reproduction circuit, which reproduces digital data from an information storage medium, and recording/reproducing that code on/from an information storage medium.
2. Description of the Related Art
As recording media that can store digital data, optical disks represented by DVDs are known. DVD-RAM (a type of DVD) comprises a signal recording layer. When this recording layer is irradiated with a laser beam of an appropriate energy, its crystalline state changes. Digital data can be recorded on the recording layer by exploiting such characteristics, i.e., a change in crystalline state. When the recording layer is irradiated with a laser light of appropriate energy, reflected light is obtained in a quantity corresponding to the crystalline state of the recording layer. By detecting this reflected light, digital data recorded on the recording layer can be reproduced.
In recent years, a PRML (Partial Response and Maximum Likelihood) technique is adopted to improve the recording density. A reference such as Jpn. Pat. Appln. KOKAI Publication No. 9-17130 or the like discloses the technical contents of the PRML technique. The technical contents of that technique will be briefly explained below for the purpose of ease of understanding.
Partial Response (PR) is a method of reproducing data while compressing a required signal band by positively utilizing intersymbol interference (interference between reproduction signals corresponding to bits which are recorded at neighboring positions). PR can be further categorized into a plurality of different classes depending on the way intersymbol interference is produced at that time. For example, in case of class 1, recorded data xe2x80x9c1xe2x80x9d is reproduced as 2-bit reproduction data xe2x80x9c11xe2x80x9d, and intersymbol interference is produced for the subsequent 1 bit. A Viterbi decoding method (ML) is a kind of so-called maximum likelihood sequence estimation method, and reproduces data on the basis of information of signal amplitudes a plurality of times by effectively using the intersymbol interference rules of a reproduction waveform. For this process, synchronous clocks, which are synchronized with a reproduction waveform obtained from a recording medium, are generated, and the reproduction waveform itself is sampled using the clocks to be converted into amplitude information. After that, the amplitude information undergoes appropriate waveform equalization to be converted into a predetermined PR response waveform, and a Viterbi decoder outputs a maximum likely data sequence as reproduction data using old and current sample data. A combination of the aforementioned PR method and Viterbi decoding method (most likelihood decoding) is called a PRML method. In order to put this PRML technique into practice, a high-precision adaptive equalization technique that obtains a reproduction signal as a response of a target PR class, and a high-precision clock reproduction technique that supports the former technique are required.
A run length limited code used in the PRML technique will be explained below. A PRML reproduction circuit generates clocks synchronized with a signal itself reproduced from a recording medium from that signal. In order to generate stable clocks, the polarity of a recorded signal must be inverted within a predetermined period of time. At the same time, the polarity of the recorded signal must be inhibited from being inverted during the predetermined period of time, so as to reduce the maximum frequency of the recorded signal. A maximum data length free from inversion of the polarity of the recorded signal is called a maximum run length, and a minimum data length free from inversion of the polarity is called a minimum run length. A modulation rule which has a maximum run length of 8 bits and a minimum run length of 2 bits is called (1, 7)RLL, and a modulation rule which has a maximum run length of 8 bits and a minimum run length of 3 bits is called (2, 7)RLL. That is, a run length limited code sequence in which the minimum run length of identical codes is (d+1) and the maximum run length of identical codes is (k+1) is called a (d, k) run length limited code sequence. As a typical modulation/demodulation method used in an optical disk, (1, 7)RLL and EFM Plus (U.S. Pat. No. 5,696,505) are known.
In general, upon recording data on an optical disk, test data is recorded/reproduced on/from a dedicated recording calibration area to adjust a recording laser power and recording pulse shape. Even when data reproduction alone is made, test data is temporarily recorded on a dedicated area as in data recording, so as to determine the equalization characteristics of a reproduction circuit compatible to a recording medium at that time, and optimal equalization characteristics are obtained by adaptive learning while reproducing the recorded signal.
As a test data (test write) pattern used in this case, a technique described in Jpn. Pat. Appln. KOKAI Publication No. 2002-15479 is known. With this test pattern, runs of 2T, 2T, 4T, i.e., runs of a [ . . . 0011000011001111 . . . ] pattern are recorded on a test data area. By reproducing this signal, the recording power is adjusted and the comparison level for Viterbi decoding is adjusted at the same time. In this manner, the recording power and Viterbi decoder can be adjusted appropriately.
Upon making modulation based on (1, 7)RLL, the recorded mark length falls within the range from 2T to 8T. However, since the test data pattern disclosed in Jpn. Pat. Appln. KOKAI Publication No. 2002-15479 above uses a test pattern of 2T, 2T, 4T, the rate of patterns with short recorded mark lengths (many high-frequency components) is high. High-frequency components contained in a reproduction signal are attenuated considerably as the recording density improves. For this reason, when adaptive learning of a waveform equalizer is insufficient, it becomes difficult to obtain stable, high-precision reproduction clocks. Consequently, since clocks are unstable, adaptive learning does not function well. If optimal waveform equalization conditions are known, no problems are posed. However, in an optical disk that can exchange recording media, adaptive learning must start from non-optimal equalization conditions.
A run length limited code generation method according to an embodiment of the present invention comprises: generating a plurality of different code sequences, which have recording densities that gradually become higher, as a plurality of different code sequences which are to be recorded on a plurality of successive subfields on a test data field of an information storage medium.
A run length limited code recording/reproduction apparatus according to an embodiment of the present invention comprises: a generation unit for generating a plurality of different code sequences which have recording densities that gradually become higher; and a recording unit for recording the plurality of different code sequences generated by the generation unit on a plurality of successive subfields in a test data field of an information storage medium.
A run length limited code recording/reproduction method according to an embodiment of the present invention comprises: generating a plurality of different code sequences which have recording densities that gradually become higher; and recording the plurality of generated different code sequences on a plurality of successive subfields in a test data field of an information storage medium.